Generally, the grain oriented electrical steel is a soft magnetic material exhibiting superior magnetic properties in its rolling direction, and this material has to be easy to magnetically excite and low in its iron loss. The exciting property is evaluated based on the level of the magnetic flux density B.sub.10 which is induced by a certain level of magnetizing force (1000 A/m), while the iron loss is evaluated by the magnitude of energy loss (W.sub.17/50) which occurs when the steel is induced to a certain level of magnetic flux density (1.7 Tesla) by an alternating current of a certain frequency (50 Hz).
A material showing a high magnetic flux density is usually used in miniature high performance electrical apparatuses, while a low iron loss means a low energy dissipation.
In a grain oriented electrical steel sheet which consists of crystal grains having an orientation of (110) [001] in the Miller indices, if the magnetic flux density and the iron loss properties are to be improved, the orientation of the steel has to be improved. That is, the direction [001], which is the direction of easy magnetization, has to correspond with the rolling direction of the steel sheet.
The grain oriented electrical steel in the industrial field is manufactured by utilizing the so-called secondary recrystallization phenomenon which occurs during the final annealing process (which is carried out at a high temperature of over 1000.degree. C.,) after cold-rolling the steel sheet to the final thickness, and after subjecting it to a decarburizing annealing.
During the secondary recrystallization, the grains having the orientation of (110)[001] devour surrounding grains having the other orientation and grow to very large sized grains.
If such a secondary recrystallization is to be producted in a perfect manner, there is required an inhibiting force which inhibits the normal growth of the primary recrystallization grains of the other orientations, during the growth of the secondary recrystallization grains.
Further, recently in pace with the increased need for the saving of energy, it is demanded that the thickness of the steel sheet be reduced in addition to the improvement of orientation in order to improve the iron loss. This is due to the fact that eddy current loss which occupies the greater part of the iron loss is proportionate to a square of the thickness of the steel sheet, and that the thinner the thickness of steel sheet is, the smaller the iron loss is. However, if the thickness of the steel sheet is made thinner, not only the secondary recrystallization becomes unstable, but also the orientation is degraded. Therefore, the lower limit of the thickness of the grain oriented electrical steel sheet which can be manufactured in a stable manner by the normal method is about 0.30 mm.
Therefore, if the iron loss is to be improved by reducing the thickness of the steel sheet, the inhibiting force against the normal growth has to be reinforced, so that the secondary recrystallization should occur in a perfect manner.
As a method of inhibiting grain growth during the manufacturing of the grain oriented electrical steel sheet, it is known that one or more of precipitating compounds such as MnS, AlN, MnSe and the like or grain boundary segregating elements are added at the melting stage, and that a precipitation treatment is carried out on the steel sheet at a later step of the process.
According to Zener's formula, the inhibiting force is defined to be .sigma..OMEGA./.UPSILON..omicron. (.UPSILON..omicron.: average particle size of the precipitates, .OMEGA.: volume fraction of the precipitations, and .sigma.: grain boundary energy). According to this formula, if the value of .UPSILON..omicron. is small, and if .OMEGA. is large, then the inhibiting force is increased. That is, if fine precipitates can be formed, a sufficient inhibiting force can be obtained with only the precipitations, the logical conclusion being so. However, in actuality, there is a limit to simultaneously achieving a large amount of precipitates and a reduction of the their size, and therefore, it should be effective to add and distribute two or more precipitating compounds or grain boundary segregating elements.
In the method for improving the orientation of the grain oriented electrical steel as described above, if a high reduction ratio is used in the final cold rolling process, the driving force for the growth of the primary recrystallization grains is increased, and therefore, a larger inhibiting force is required.
For example, a magnetic flux density of about 1.8 Tesla is obtained by carrying out a cold rolling process using a reduction ratio of 60% in one of the conventional oriented electrical steels. In such a case, MnS precipitates are used as main inhibitors. On the other hand, in another oriented electrical steel in which a magnetic flux density of 1.90 Tesla is obtained by carrying out a cold rolling process using a higher reduction ratio of over 80%, two or more of precipitating compounds such as MnS and AlN are used as the inhibiting agents.
Further, according to Japanese Patent Publication No. Sho57-45818, the grain growth inhibiting force is reinforced by adding Cu as a sulfide forming element in addition to MnS and AlN, and a reduction ratio of 87% is applied, thereby providing a process for manufacturing a grain oriented electrical steel sheet having superior magnetic properties.
Meanwhile a process of adding P in the melting stage of the grain oriented electrical steel is disclosed in Japanese Patent Publication No. Sho-52-6329. By adding P, the precipitates such as MnS and AlN can be more uniformly distributed in the form of tiny particles, and consequently, the secondary recrystallization grains become more fine, thereby improving the iron loss properties. However, if the effect of the addition of P is to be obtained, Ni has to be necessarily added, and, if its addition is less than 0.03%, the secondary recrystallization becomes unstable.